The mechanism of sister chromatid recombination is poorly understood at the genetic and molecular level. Since it may be important in repair, mutagenesis and gene amplification, we have initiated studies to characterize the process and investigate the genetic control in the yeast Saccharomyces cerevisiae. Sister chromatid recombination is measured by recombination between two fragments of the HIS3 gene which share a 300 base pair region of homology. They are arranged in such a way that recombination will generate a complete gene. We have shown that several of the genes involved in DNA repair are required for the levels observed in wild type cells. Mutations in genes that enhance or depress homologous chromosome recombination proces. DNA damaging agents can considerably enhance the process although the cells are resistant to x-ray induced exchange. In order to determine more accurately the levels of spontaneous sister chromatid recombination, we have developed procedures for greatly reducing the background levels. This is allowing small increases to be determined more accurately.